The present invention relates to supercharger bypass valves and in particular to an external spring used to alter closing of the supercharger bypass valve.
Power production of an internal combustion engine is ultimately limited by the amount of air introduced into each engine cylinder. Fuel systems can at best provide an optimal amount of fuel to burn with the air contained in the cylinder, and adding more fuel than required for a stoichiometric air-fuel ratio does not result in more energy being produced. The power production of non-supercharged engines is thus limited by the engine's ability to draw air into each cylinder, referred to the Volumetric Efficiency (VE) of the engine, where 100 percent VE is equivalent to complete filling of the cylinder at bottom dead center at one atmosphere of pressure. While some engines achieve greater than 100 percent VE using tuned intake manifolds, the effects are generally limited to a small RPM range which the intake is tuned to.
Power production may also be realized by raising the RPM that an engine is operated at, thereby pumping more air through the engine. Unfortunately, high RPM operation requires cam lobe designs which are inefficient at low RPM, and is also stressful on engine parts.
An alternative method for increasing power production is to pump (or force) air into the engine. This approach is commonly called supercharging because more air is forced into each cylinder than 100 percent VE produces. For many years, supercharging was limited to special applications because of the power required to operate the supercharger (i.e., the parasitic draw of the supercharger) resulting in reduced fuel economy under all operating conditions.
One known supercharger is a screw compressor type supercharger employed to pump air into the engine at greater than atmospheric pressure to increasing horsepower. Screw compressor superchargers employ a pair of rotating screw elements within a confined cylindrical housing. The rotating screw elements draw air from a throttle body at an aft end of the housing and push the air progressing toward a forward end of the housing thereby compressing the air. The compressed air is then delivered into an intake manifold of the internal combustion engine. Providing the compressed air (commonly referred to as boost) dramatically increases engine horsepower production and allows immediate and tremendous acceleration.
However, maximum performance is not always required or desired from an engine and is only infrequently required from the engine in a street driven vehicle. The pressure boost generated by constantly running a supercharger elevates intake air temperature that causes ill effects on the engine, performance, fuel economy, and emissions. Therefore, for normal driving, it is ideal to effectively disconnect the compressor, but unfortunately, known means for selectively engaging and disengaging a supercharger are not cost effective.
As a solution to the ill effects of a constantly running supercharger, both original equipment and aftermarket supercharger systems have been developed which selectively bypass the compressed heated air flow back to the supercharger inlet during non-performance driving. Such bypassing eliminates the negative effects of the supercharger at low speeds while still providing a highly efficient method for producing significant horse power gains. Such bypassing further maintains reasonable and often provides improved fuel economy. The control of such bypassed air flow is commonly performed by a bypass valve. Unfortunately, known bypass valves work well at low boost pressures common in production cars, for example, below 6 PSI boost (or MAP of 20.7 PSIA), but not always satisfactory for high boost pressures, for example, from 6 to 20 PSI (or MAP from 20.7 PSIA to 34.7 PSIA) of modern high performance superchargers.
Further, modern vehicle engines provide significant improvements in speed, economy, and emissions through the use of computer controlled Electronic Fuel Injection (EFI) systems. The EFI systems measure engine parameters and determine how much fuel to provide to the engine for efficient operation. Known EFI systems fall into three categories Alpha N systems; Speed Density systems; and Mass Air Flow systems.
Alpha N systems tend to be simple and compute fuel requirements based on RPM and throttle position. The RPM and throttle position are provided to simple lookup tables and the fuel requirement results. Alpha N systems often work well in racing engines operated at wide open throttle, but are difficult to tune to a wide operating range.
Speed Density systems receive engine RPM, throttle position, intake manifold vacuum, and intake air temperature, and compute airflow requirements using a much larger lookup table than an Alpha N system. Some Speed Density systems also include an oxygen (O2) sensor in the exhaust system to provide closed loop operation. In closed loop operation the system uses the air/fuel ratio from the O2 sensor to adapt to current conditions and adjust for engine wear. Some General Motors and most Dodge fuel injection systems use Speed Density systems.
Mass Air Flow (MAF) systems include an MAF sensor mounted in front of the throttle body which directly measures the amount (mass) of air inducted into the engine. A known MAF sensor is a hot wire sensor. Air flows over a heated wire and draws away some of the heat. The amount of current required to maintain the temperature of the wire is measured and used to determine the mass of air flowing across the wire. The mass of air flow, plus additional sensor data, is input to a map, and fuel requirements determined. The MAF systems offer good performance by directly measuring the air flow, but the required air flow sensor creates a restriction in some systems and reduces performance.
Chrysler Speed Density systems include Drive-By-Wire throttle position control and are designed to operate in a Manifold Air Pressure (MAP) range of approximately zero to 15 Pounds per Square Inch Absolute (PSIA) (where one atmosphere equals approximately 14.7 PSIA.) As a result, the software running the Chrysler Speed Density system does not lend itself to supercharged applications where MAP can exceed 15 PSIA.
More specifically, the Chrysler throttle position is controlled by software via a MAP sensor output which is translated to an airflow estimate by the Speed Density system and the airflow value is translated into a torque estimate. Under normal driving conditions, throttle position is controlled based on demand from the driver in the form of pedal position, but is also limited by a lookup table based on instantaneous torque estimates. When the driver advances the pedal position, the software limits the actual throttle position based on the lookup table. The system assumes that if the MAP sensor output reaches 14 PSIA to 15 PSIA, the airflow into the engine is at a maximum independent of throttle position, and the software no longer limits the throttle position. Under normally aspirated conditions (no boost), there is no reaction to this, because once the intake manifold absolute pressure is near or equal to one atmosphere, greater throttle position has no affect on air flow into the engine. The software thus allows throttle position to match the pedal position once one atmosphere is attained (approx 14.7 PSIA).
Unfortunately, such throttle position control produces undesirable results for supercharged (or boosted, pressures above one atmosphere) applications. Because the Chrysler software immediately allows the throttle position to match pedal position once the MAP sensor outputs reach one atmosphere, the unexpected increase in throttle position compounds the effect of boost when boost is created. In other words, there is an rapid transition from limited throttle position to unlimited throttle position (throttle position is now commanded to pedal position) as soon as the MAP sensor output increases to above one atmosphere because the Chrysler software incorrectly assumes that no boost is present.
Additionally, the bypass valve present on known superchargers rapidly closes over the same MAP range as where the throttle position is allowed to open rapidly, causing boost to increase even more rapidly. An example comparing throttle voltage (proportional to throttle position) and MAP with a standard “pre-loaded” bypass valve (STD bypass) to an throttle voltage and MAP with an improved bypass valve according to the present invention and discussed below is shown in FIG. 10. The MAP (boost) increases rapidly as the STD bypass valve closes at just below one atmosphere MAP (i.e., just before the engine transitions from vacuum to boost) just as throttle position limits are removed. This compounded application of increased throttle position and increased boost produces an undesirable “ON/OFF” feel to the driver.
Unfortunately, the Chrysler software does not have a provision to allow for a smooth transition of throttle position control above 14.7 PSIA, so it is necessary to find another way to attenuate the instantaneous increase in power being experienced with supercharged engines. Many solutions have been tried over a period of five years, including switches on the pedal, and other, but no simple, robust, solution has been found, and Chrysler engineers have failed to provide a solution to this problem.
Further, both Ford and GM Mass Air Flow (MAF) systems using STD bypass valves in conjunction with race or high lift, high overlap cams, may cause overheating of the supercharger because the STD bypass valve closes prematurely. The STD bypass valve is almost fully closed at 10 inches Hg (i.e., Map of 9.8 PSIA). Most race or high lift, high overlap cams reduce vacuum to a maximum of 10 inches Hg or less (i.e., a MAP of 9.8 PSIA or more) during cruising even where throttle positions are very low. This vacuum level forces the STD bypass valve to remain closed during most operation, often overheating the supercharger and causing premature failure.